The transition from DeVries-Rose to Weber's laws: comments on Rovamo, Mustonen and Näsänen (1995).

Rovamo et al. [Vision Research (1995), 35, 767-774] measured contrast sensitivity at several frequencies in the fovea and periphery as a function of retinal illuminance, concluding that the critical illuminance for the transition from DeVries-Rose to Weber's laws is proportional to squared frequency at all retinal locations. Yet, inspection of their data clearly reveals that the DeVries-Rose range was hardly ever followed by a Weber range: either no sign of any second range was apparent or the transition was to a qualitatively different range in which sensitivity decreased with increasing illuminance. The validity of their conclusions is questioned, and the status of the "DeVries-Rose to Weber transition" as a description of the relationship between sensitivity and illuminance is discussed in the light of mounting empirical evidence of a decreasing range in this relationship.